The present invention relates to substrates for superconductors, and more particularly to copper-nickel substrates for receiving the deposition of YBCO (YBa2Cu3Ox, or Yttrium-Barium-Copper-Oxide) high temperature superconducting layers to form so-called YBCO coated conductors. Other closely related superconducting materials which can be used are REBa2Cu3Ox, in which the Y has been partially or completely replaced by rare earth (RE) elements.
YBCO (Yxe2x80x94Ba2xe2x80x94Cu3xe2x80x94Ox) is an important superconducting material for the development of superconducting tapes that can be used in superconducting transmission cables, superconducting current leads, superconducting coils for transformers, superconducting magnets for AC and DC motor applications, and superconducting current limiters, as well as other electrical conductors. These applications are based on a basic property of a superconducting material: it has no electrical resistance when cooled below its transition temperature, and can carry a DC electric current without power dissipation.
For the production of YBCO coated conductors, thin substrate tapes (or foils) are typically coated with a thin buffer layer, which in turn is coated with a YBCO layer. A suitable heat treatment is then performed to optimize the superconducting properties of the YBCO layer. One of the functions of the substrate is to impart mechanical strength to the resulting superconducting tape. A second function, which depends on the process type, is to act as a template for a well-textured buffer layer. Compared to the substrate material, this buffer layer provides a much better deposition surface for the YBCO layer in terms of lattice match, texture, coefficient of thermal expansion (CTE) and chemical compatibility. To obtain good superconducting properties, the buffer layer needs to be bi-axially texturedxe2x80x94meaning that a lattice plane, preferably its cubic (or tetragonal) lattice face, is oriented such that the cube face is parallel to the tape surface in a substantial majority of its crystallites. In addition, the cube edge in each crystallite should be parallel to the cube edges in all neighboring crystallites.
Some specialized techniques such as Ion Beam Assisted Deposition (IBAD) or Inclined Substrate Deposition (ISD) can deposit a bi-axially textured buffer layer on top of a random polycrystalline or even amorphous substrate. In general, these deposition techniques are very slow or are effective in only a narrow region, and they are not suited for large scale and economical manufacturing of YBCO coated conductors. A more advantageous method is the epitaxial deposition of a bi-axially textured buffer layer (or YBCO superconducting layer) on top of a bi-axially textured metallic substrate. Examples of epitaxial growth by vapor deposition, electroplating, or oxidation, in which native oxide layers grow on parent metals, are numerous and well known, as is the fact that many metals can form bi-axial textures. Few of these textures are useful for deposition of buffer layers and YBCO superconducting layers because of misorientation. However, in many rolled, face-centered cubic (fcc) metals, when properly rolled and heat treated, a well-developed, and very useful, cube texture is obtained. The cube faces are parallel to the rolled surface and a cube edge typically points in the same direction as the rolling direction. Such a texture is called a cube-on-cube texture, with a crystallographic notation of (100) [001]. Another well-known cube texture is the so-called Goss texture (100)[011]. These bi-axial textures are part of a larger family called sheet textures. In the following description of the invention the (100) [001] texture will be referred to as the xe2x80x9ccubexe2x80x9d texture.
To deposit the buffer layer in an epitaxial manner on the substrate, the substrate material needs to meet certain requirements. The substrate must have a lattice constant and a CTE which are compatible with the buffer layer material and also with the YBCO layer. Ideally, the substrate will yield a bi-axial texture by simple thermo-mechanical means. The substrate is preferably non-magnetic at cryogenic temperatures, that is, at temperatures between room temperature and that of liquid helium, or 4.2 degrees Kelvin. The substrate must be electrically conductive, relatively strong at room temperature, and oxidation resistant at elevated temperatures. There are several metals, such as copper or nickel, that can be bi-axially textured by rolling a selected copper or nickel stock, followed by a so-called secondary recrystallization at an elevated temperature. However, these pure metals have significant drawbacks in that they are either ferromagnetic (Ni) or are easy to oxidize (Cu).
It is known that some binary alloys (a single phase mixture of two metals) can be made into a bi-axially textured tape as well. One example of a cube texture which has been produced in an alloy is iron-nickel, but this alloy has proven to be ferromagnetic, which is detrimental to the performance of the device in many applications. In addition, copper-nickel alloys with small quantities of nickel have previously been textured, but those working in the field believed that the maximum Ni content in the Cuxe2x80x94Ni alloy should not exceed 4.2 percent nickel.
The present invention features bi-axially textured alloys with a face centered cubic structure, of copper-nickel (Cuxe2x80x94Ni) with 5 to 45 atomic % nickel, preferably 10-40% and more preferably 25-35%, for use as substrate materials for superconducting oxides. Preferred superconducting oxides include the Rare Earth Barium Copper Oxides, (RE)BCO, or Yttrium Barium Copper Oxides, YBCO, but also superconducting oxides from the Thallium, Mercury and Bismuth families. The combination of the substrate and the oxide forms a coated conductor. These Cuxe2x80x94Ni alloys are preferably homogenous, but can have some degree of inhomogeneity with localized concentrations of Ni not exceeding 45%, and can be processed by thermo-mechanical methods to form tapes with a single (100) [001] cube texture. These alloys are non-ferromagnetic and form good substrate materials for subsequent epitaxial buffer layer and superconductor layer deposition, for use in a variety of products.
The enhanced Ni content achieves desirable features in the alloy, such as increased oxidation resistance, decreased CTE, and increased room temperature and high temperature strength. The increased Ni content does not cause ferromagnetism as long as the Ni content does not exceed 42% for applications down to 4.2 degrees Kelvin or 45% for applications at temperatures between 4.2 and 77 degrees Kelvin. With the appropriate buffer layer material, this substrate will not adversely affect the superconducting properties of the superconducting layer.
Cube-textured alloys of the present invention are formed by creating a homogenous solid solution of the alloying elements. The Cu and Ni constituents are weighed, mixed, and melted together to form a CuNi alloy. The starting materials are preferably at least 99% pure. The melt is then cooled to room temperature. The rate of cooling can be slow or fast, with a rapid quench preferred for giving a fine grain size. The solidified alloy is further homogenized by a heat treatment. The alloy is then processed into tape by mechanical means, such as rolling, after which a suitable heat treatment produces the desired cube texture. An optional recrystallization step after the homogenization and partial deformation of the alloy induces a refined grain size of 5 to 70 micrometers, which obtains a cube texture in the rolled and heat treated tapes.